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Voltage reduction

Andy Betts

i build guitar effects pedals for a hobbie, and I have a modification I'd like to carry out and i'm hoping some of you may be able to guide me with a solution.

 

Ok... Here's my plan. I have 2 effects pedals... An electric mistress which runs on 18v (it actually has a voltage regulator inside it with reduces the voltage to around 13v but it does run on 2 9v batteries), the second pedal is a boost pedal which runs a single 9v battery which i currently run after the electric mistress to boost the output. The problem with the electric mistress is when you engage the effect the output volume drops by around 10 to 15%, so the boost pedal compensates for that drop.

 

What i want to do is build the boost circuit into the electric mistress enclosure.. so the guitar will go into the electric mistress travel through the mistress circuit to the output of the circuit then into the boost circuit, then to the output jack socket. Simple, but i have a dilema inso far as they each run on different voltages, so how can i regulate the 18v DC down to 9vdc for the boost part of the circuit? If they both ran on the same voltage it'd be easy.... but i'm a little lost with this.

 

I've been told to use a resistor but I've also been told that's not the right way to do it.... Any ideas?

Parents Reply Children
  • in reply to jw0752

    Using a dc-to-dc converter should be the preferred option as it is more efficient. Dropping 18 V to 9 V using a linear regulator means loosing half the power to the heat sink.

  • in reply to afmishaq

    Have a look at the efficiency of this cheap switching circuit at 10mA. It probably isn't much better than using a linear regulator.

  • in reply to jw0752

    This is audio equipment. Batteries are nice and quiet. Why spoil things by putting a switcher in the same box?

  • in reply to jc2048

    Well I also thought about.

    But high frequency switcher does not interference to the signal.

    Please look into a modern microphone also in there are switcher supplies build in.

    E.g. AKG microphones have booster with three output voltages, boosting from 1.5V battery.

    They are very small as well coming with optimized power consumption.

     

    Best Regards

    Gerald

  • in reply to geralds

    OK, I'm wrong then (or perhaps just too old and behind the times). Thanks for putting me right.

     

    But those are very specific to the job and are carefully designed and tested in a way that Andy isn't going to manage. I'd still contend that it's a mistake encouraging him to use a cheap switcher like the Recom devices.

  • in reply to jc2048

    Yes, I agree with you completely.

     

    For him it would be good to use LDOs with very low dropouts.

    And this LDOs would be selected in seeing the electrical current consumption.

    Then may be, then it wouldn't needing a heat sink.

    Regulating via Zener is very inefficient.

  • in reply to jc2048

    Hi Jon,

    I will investigate and compare efficiencies this evening.

     

    John

  • in reply to jc2048

    Hi Jon,

    I have run a three simple tests and here are the results.

    I have Breadboarded an LM7809 on the left of the breadboard and a Recom R78C9.0-1.0 on the right side. Both have been provided with input and output capacitors as specified in their data sheets. The power source for this experiment is a series bank of (3) 6.3 volt lead acid gel cells for a total of 18.9 volts. Hopefully this will eliminate power supply fluctuations as a variable.  All meters and loads have been set up left to right to correspond to the relative positions of the DUTs. The Radio Shack and Fluke are set up to read mA with the RS displaying input current for the LM7809 and the Fluke displaying current for the R78C9.0. I was very happy to have a good use for my two new low current loads with the left unit displaying the load on the 7809 and the right unit displaying the load on the R78C9.0. The oscilloscope has the yellow channel 1 on the output of the 7809 and the blue channel is on the output of the R78C9.0. Both oscilloscope channels are set to 20 mV vertical resolution and AC input with the voltage displayed for volts P-P. Here is a picture of the experimental layout.:

     

    image

     

    If you click on the picture you can expand it enough to see most of the details except the scope. I ran three tests, one at 5 mA, another at 10 mA load and finally one at a 20 mA load. Here are the results:

     

    5 mA Load Test

                           7805                            R78C9.0

    V Input            18.8 V                           18.8 V

    I Input               8.7 mA                        3.83 mA

    V Output          8.9 V                            9.1 V

    I Output            5.0 mA                         5.0 mA

    Efficiency         27%                              63%

     

    10 mA Load Test

     

    V Input           18.9 V                          18.9 V

    I Input             13.7 mA                         6.31 mA

    V Output           8.9 V                           9.1 V

    I Output           10 mA                          10 mA

    Efficiency         34%                             76%

     

    20 mA Load Test

     

    V Input             18.8 V                         18.8 V

    I Input              24.2 mA                       11.59 mA

    V Output            8.9 V                            9.1 V

    I Output           20 mA                           20 mA

    Efficiency         39%                              83.5%

     

    Now for the rest of the story. As Jon pointed out the cost for this efficiency is some noise from the R78C9.0 converter. Here is a picture of the oscilloscope:

     

    image

    The LM7809 has noise with spikes of about 6 mV PP and the R78C9.0 has spikes of about 22 mV PP. I tried to get a frequency read on the R78 noise and the best I could see was a range of 50 kHz to 100 kHz as the reading jumped around. It is noted that this is well about audio frequencies and may not affect audio performance.

     

    It seems to me that the difference in efficiencies may double the life of the battery.

     

    I will leave the test set up for an hour or so in case anyone want me to run any further experiments.

     

    John

  • in reply to jw0752

    Thanks,

    This confirms my explanation in the posting to Jon Clift.
    Whereby, the LM7809 is rather a not so good regulator, as it is in the meantime on the market.

    In addition, the own noise of the oscilloscope is added.

    (In your table you wrote 7905 instead of 7809)